2013 US Solar Performance Study Shows Reduced Production in Major Project Zones

Surprising Anomalies Underscore the Value of Solar Assessment and Performance Reconciliation

SEATTLE (October 9, 2013) – 3TIER®, a global leader in renewable energy assessment and forecasting, today released solar performance maps of the Continental US for June, July, and August of 2013. The maps are the result of an anomaly study, which illustrates how solar irradiance varied from its long-term norm, and the maps show strong correlations with a series of weather events that affected the US this past summer. The findings demonstrate that solar energy is not immune to climatic variability, the risk of which needs to be factored into the financial structure of projects.


"One surprise of the 2013 solar performance study was that areas of the country typically associated with summer sunshine had below normal irradiance, while the Pacific Northwest, often stereotyped as cloudy and rainy, had one of the clearest summers on record," said Dr. Mark Stoelinga, senior scientist at 3TIER. "This underlines the fact that solar project underperformance is a risk – even for sites in sunny, desert regions during peak production months."

Solar irradiance variances caused by weather anomalies have a substantial impact on the profitability of a solar power project. Prior to development, utility-scale projects must complete a comprehensive assessment of the solar resource, including on-site measurement and a long-term evaluation of interannual variability. However, solar plants already in operation also need to put above or below normal production into a long-term context to reconcile performance and make numerous decisions about the project's functionality.

"The massive installations now online in the southwestern US as well as the tight cluster of projects along the Northeast, definitely saw reduced production this summer. And for some of them, it was only their first year of operation, which makes it particularly important to understand why output was lower than expected," said Gwendalyn Bender, energy assessment product manager at 3TIER. "Since the equipment is still under its first year manufacturer's warranty, determining whether underperformance was caused by weather or by panels and inverters is critical."

Findings from the 2013 three-month study include:

June was characterized by a nationwide east-to-west gradient in insolation, from above normal in the west to below normal in the east. Unusually clear weather occurred along the coastal Pacific Northwest and most of the West was gripped by drought conditions throughout much of the month. The lack of cloudiness was due in part to a teleconnection with a cyclic thunderstorm complex in the tropical Pacific Ocean known as the Madden Julian Oscillation.

July saw lower than normal insolation for much of the country, most notably in the Southwest Desert where solar irradiance is typically high. This was partially in response to a very hot, clear June within that region, which contributed to a particularly strong North American Monsoon, leading to more precipitation and cloudiness beginning in early July. A strong exception to the national pattern was seen in the coastal Pacific Northwest, which, as in June, again saw uncharacteristic clear, sunny weather.

August continued the cloudier than normal trend over the Southwest as the strong North American Monsoon persisted, but a complex pattern of cloudy and clear anomalies developed over the remainder of the US. Clear conditions prevailed over the northern Rockies, southern plains and upper Midwest, while cloudier conditions were experienced in the Southeast and along the West Coast. A strong localized negative anomaly can be seen in northeast-central California, particularly noted in the DNI map, which was associated with the Rim Fire near Yosemite Park.

3TIER's solar performance analysis is based on its global solar dataset, which provides continuous, hourly records of irradiance for 15+ years at any location in the world and is updated monthly for most regions. 3TIER used this dataset to determine long-term average irradiance and to calculate conditions for the summer of 2013. The resulting maps illustrate departures from the long-term average that range from -20% to +20%.

For more information on how 3TIER can help you make informed decisions about past and future project performance, please meet with us at Booth #956 at Solar Power International in Chicago, October 21-24, or visit www.3tier.com.

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